Circuit for amplifier tubes



Aug. 18, 1931. R. H. HOLMES CIRCUIT FOR AMPLIFIER TUBES Filed March 18, 1925 ITIII INVENTOR Fa fia/mes.

WIT ESSES:

ATTTORN'EIY Patented Aug. 18, 1931 1 UNITED STATES; rerun-r eerie-e;

BAY H.- .HQLMES, F WI KlNSBUB-G, .lflNusyhvnn -e, ASSIGNOB no wnsrmezzoosn ,EJ ECL HQ Ml UFAQTUBINcG GQMPANY; QQBT QRA'S QN 92 E T- CIRCUIT FOB- .AM'ELTFIER TUBES Application ifled. Marsh 318,1825. Serial No. 16 345.

This invention relates to vacuum d vices. lhllone particularly, it relates to 2; X219- num tube system in which provisionis made :fior preventing the tubes from oscillating.

s it is an object of this invention to avoid the 5-955 of sensitive-noes -heneto iore caused in such systems by currents of .aireqnenov ab eve that at which the tuned rcuit is inte d d to ope ate v oscillations oi higher f rsqu nci are ordinarily termed n rasitieWenrr-enis They a e usually caused by the pizesence oi harmonies in an os illating They m y, we, r s -1 m ot e cau es, some of which are des ibed b low It is a fi-urther obiwt fi i wsm ento provide on absorption circuit in connect on :witlreafih mined circu t he e rer siiic currents are likely to occur,

:The parasitic u r ts allowed to oeeur are likely to block the tube completely or least render it less sensitiwe,

In the system to which this inweuiisn preferably applied, an oseilletin-g circuit provided comprising an id diuctor and a condenser in parallel. The inductor has s {top at a point int rmediate its ends- T ih fi d' ca-lly, IfOP the best operation of the system, the magnetic coupling between th 3 9 parts ofathiscoil on opposite sideslof the tap should be 100%. Such complete coupling is possible and the incompleteness .of the (coupling tends to produce 1 oscillation Whwh is of considerable amplitude and usually para-lyzes the amplifier tub be zmse of relatin-sly large alternating current volt ge impressed on its grid.

It is a -f-uuther object of this innentzion to preven sc lla ons i th s i il itic frequency from being deneloped.

My invention is profitably used with a receiw'ing set having several stages of readiefrequency amplification, each of said stages having a tuned grid Q'hfiEHdt and also a neutralizing condenser. The condenser pneven-ts the regenena hion from becmning suiflieient to cause the tubes to-osoillste, It is a further object of my invention to placid-(1e, in each of said stages, a absorption circuit which will co ntered any endeesgz te sitic oscillations due to the fact that the two fiail lii *Qi 16 inductor in the grid circuit are not perfectly coupled.

It is a still further object of my invention to snake the inning oi'the absorption circuit :brond, thereby providing for the difierences AI} frequency of the parasitic oscillations in difieren-t i ,rbreadth of the .tumng of the absorption .circilit also prevents beat notes which might otherwise aI'iSQHfiOIH the interaction of, two :frequencies, one impressed on the by the received energy and the other due to. the response of the tuned circuit bhem sm The broad tuning of the absorpt on circuit also prevents periodic inter- ;Qf energy between it and the tuned cineuit which-might give rise to noises in the tub s;

.Qtherphjects of vmy invention and details of lthfiQQllStPllfitlQIl will be apparent from the gfiollowing description, when read in conneci ion with the accompanying drawing, where n The single figure is a diagram of the cir-' cu-its and apparatus constituting a indio re- (eeiving system which a preferred form of n inwention is applied.

The antenna 1 is connected by the switch 2 $0 1 6 e lld-l s t0r 3, either directly or through thecondexnser 4;. The mid point of the inducter 3 .isrconneoted to ground through the condenser .5. The inductor 3 is shunted by an adjustable condenser "6, the inductor and con- ;denser together constituting a tuned circuit forms part of the grid circuit of the Vacuum tube't'. The plate circuit of the tube 7 includes the primary 8 of a transformer confifit fi-lg the {radio-frequency stage including he tube Tincascade with the stage including hhe tube 9.

A. battery 11 is in series with the primary coil 8, The connection from this batterp .to the filament of the tube 7 is made by means of a-zs-hicld, represented by the ground 12. A condenser 13 is shunted about the battery 11. This is represented in the drawing by showing the condenser as connected to the ground 14, which, in practice, would be the same shield. The ground 15 is associated with the filament heating circuit including the battery 16 and the rehostat 17.

The secondary 18 of the inter-stage transformer is shunted by an adjustable condenser 19. The tuned circuit 1819 is a part of the grid circuit of the tube 9. A conductor joins the mid-point of the secondary 18, "in the stage of radio-frequency, amplification --in--- cluding the tube 9, and the midpoint of the inductor 3, in the stage including the'tube 7..

The plate circuit of the tube 9 includes the primary coil 20 of a transformerconnecting this stage to the next stage. The next stage may, if desired, include another amplifier, but, as illustrated, it comprises the tube 21 which is a detector. It will be obvious, however, that as many stages'of radio-frequency amplification as are necessary may be introduced ahead of the detector.

In each of the amplifier stages,'a condenser is introduced between the oscillating or tuned circuit and the plate circuit. Preferably, this condenser is connected to that side of the oscillating circuit which is remote from the grid and that side of the plate-circuit inductance which is adjacent the plate. Thus the condenser 22 is connected to the upper terminal of the primary coil 8 as illustrated and to the lower terminal of the inductori3 and condenser 6. Likewise, the condenser 23 is connected between the upperterininal of the primary coil 20 and the lower terminals of the secondary 18 and the condenser 19..

The detector tube 21 has in its grid circuit a condenser 24 shunted by a resistor 25, constituting a grid biasing device. The filament of this tube is connected to the secondary 26 of the transformer between tubes 9 and 21. The point at which this connection is made is preferably intermediate the ends but not necessarily the middle of the secondary 26. It is so chosen that the impedance between the grid of the tube 21 and the filament shall cause minimum losses.

The connection to the secondary 26 does not include any C battery, because'the necessary bias is provided by the condenser 2s and resistor 25. The other tubes are each biased by the common C battery 27, which is shown as connected between the ground 28 and the conductor connecting the mid-points of inductor 3 and secondary coil 18. 'It will, however, be obvious that the battery 27 may be connected between the ground and any other point of the system of conductorsextending to the several grids. The grid circuit of the detector tube 21 is tuned by means of a condenser 29 in shunt with the secondary coil 26.

Preferably, provision is made for operating the filament of the tube 21 at a different temperature from that of the amplifier-tube laments. Therefore, a rheostat 30 is provided which controls the current through this filament only, the current through the other filaments being controlled by the rheostat 17.

The plate circuit of the detector tube includes the primary coil 31 of an audio frequency transformer and a portion only of the batterygll; Inorderthat a" shunt for the high nequene currents-"may be provided about-this part of the battery, a condenser 32 is. provided betweentheshield and the terminalo'f. the, coil 31 nearest the plate, The secondary of the audio-frequency transformer is connected to the grid of the tube 33 and to the negative side of the battery 27. The

.;plate circuit-of the .tube 23 includes a telephone 34 or other translating device and the whole o'f-the battery 11. Preferably the telephone 34 is shunted by a condenser 35.

" in the operation of the device, signals are received upon the antenna' l'and cause the "circuit 3'6 to'oscillatei This circuit is tuned to the frequency of the -incon1ing'signals. 'T he oscillations thereof areamplifie'd by the tube 7 and repeated in the primary 8. Any tendency of the tube 7 to oscillate at the working" I frec'i ueiicy because of a the inductive reactance of its plate circuit, is neutralized 'by'the condenser ease the divided coil 3.

The'coupling 'be'tweenthe' twoparts of the coil'3 i's not100'% If it iwere; th' e' coil would present no reactance' =to the electromotive force impressedthereon through the grid- -plate'-'capacity of the tube 7,- atone end of the -coil"3,-and through the condenser 22 at the other end the reo'f."--The oscillation 'of this eoil, if the"coupli ng'"were' perfect, would,-

therefore;'be determined wholly by the -incoming signal and -the tuning effect of the condenser 6. The form of the oscillations in the coil-3' underthese conditions canbe describedwith a fair degree OfGXa-CCQGSS as a 'voltage loopextending the length ofthe coil. The frequency of theseosci-llations is determined mainly by the constants of the circuit 3-6.' The distributed capacity of the tion. a I

On the "otherhand,-when the coupling between the t'wo halves of the coil 3 is imperfect, the-disturbances 'in the grid circuit consequentupon the inductive character of the plate circuit will not be wholly neutralized by the connections including the condenser 22; These'disturban'ces tend to set up para sitic'oscillat-ions of a very high frequency. The parasitic oscillations can be approximatelyrepresented by two voltage loops, the voltage node between them being at or near the middle of the coil.

For very high frequencies,the condenser 6 is practically a short circuit;- The two halves of 'the-coil-3, can, therefore, be regarded as connected in parallel and the two capacities, namely, the grid-plate capacity of the tube 7 and the capacity of the condenser 22, may also be regarded as connected in parallel.

coil 3 plays but little part inthis determina- The parasitic oscillations have their frequency determined mainly by the distributed capacity of the two halves of the coil 3 and by the leakage inductance, which the halfcoils possess because the coupling between them is not perfect. All of these reactances being very small, the frequency of these oscillations is high. It is, therefore, easy to selectively tune a circuit, whereby it will not be resonant to currents of the signalling frequency, but will absorb oscillations of the parasitic frequency. The difference between the parasitic frequency and the signalling frequency will even permit the absorption circuit to be so broadly tuned that it will provide for manufacturing tolerances and prevent the tube noises explained above.

The coil 41 and the condenser 42 are designed to constitute such a selective absorption circuit for the stage of the tube 7. A resistance 43 is provided in circuit with the inductor 41 and condenser 42. The absorbing effect of this circuit can be adjusted by varying the coupling of said circuit and coil 3. An approximate tuning of the absorption circuit may be provided by correctly choosing the magnitude of the inductance and capacity. The magnitude of the resistance 43 serves to determine the degree of damping. It is desirable that this damping be no greater than just sufiicient to prevent the development of oscillations of the various parasitic frequencies. Although the resistance 43 is illustrated as a separate element, it will ordinarily be provided by selecting wire of the desired resistance for the coil 41.

Similarly, an inductor 46, a condenser 47, and a resistance 48 may be provided in connection with the amplifier tube 7. If more than two stages of radio frequency amplification are employed, a similar absorption circuit may be provided for each stage.

Other applications of the means herein disclosed for preventing parasitic oscillations will readily occur to those skilled in the art. The specific illustration and description of a single application is not intended to restrict the invention. No limitation is intended other than such as are required by the prior art or indicated in the accompanying claims.

I claim as my invention:

1. In a radio device, a tunable circuit having provisions for preventing feed-back over the tuning range thereof, and absorption means for preventing parasitic oscillations therein at frequencies above said range.

2. In an amplifier, a plurality of stages, a Vacuum tube in each stage, an oscillation cir cuit in the input of each tube, said circuits being tunable to the same frequency, and absorption circuits, one for each of said tunable circuits, each of said absorption circuits being tuned to a frequency substantially the same as that of a predominating parasitic frequency of the stage with Which the said absorption circuit is associated.

3. In an amplifier, a plurality of stages, a vacuum tube in each stage, an oscillation circuit in the input of each tube, said circuits being tunable to the same frequency, and absorption circuits, one for each of said tunable circuits, each absorption circuit being tuned to a frequency several octaves above any frequency to Which its associated oscillation circuit is intended to be tunable, said higher frequency being the predominant parasitic frequency of the stage with which the absorption circuit is associated.

4. In an amplifier, a tunable amplifying stage, means for preventing said stage from oscillating at frequencies Within the range over which it is designed to be tunable, and an absorption circuit, for said stage, said absorption circuit being tuned to a predominant parasitic frequency of its associated stage, which frequency is above said range.

5. In a radio device, a triode, a grid circuit, a plate circuit, an inductor in one of said circuits, means connecting a mid-point of said inductor with a point common to both circuits, and tuned absorption means associated with said inductor for preventing parasitic oscillations therein at ultra high frequencies.

6. In a radio device, a triode, a grid circuit, a plate circuit, means for tuning one of said circuits, an inductor in one of said circuits, means connecting a mid-point of said inductor with a point common to both circuits and tuned absorption means associatedwith said inductor for preventing parasitic oscillations therein at frequencies above the tuning range, and a device between said circuits for preventing oscillations at the frequency for which one of the circuits is tuned.

In testimony whereof, I have hereunto subscribed my name this 14th day of February,

RAY H. HOLMES. 

